Fan control circuit and system

ABSTRACT

A fan control circuit for controlling a two-wire or three wire fan in a fan control system includes: a rotational speed detecting module, for detecting a rotational speed of the fan, in order to generate a rotational speed signal; a rotational speed converting module, coupled to the rotational speed detecting module, for converting the rotational speed signal into a first voltage signal; and a feedback control module, coupled to the rotational speed converting module, for generating a fan control signal to control the fan according to the first voltage signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fan control circuit and system, and more particularly, to a fan control circuit and a related fan control system for controlling a two-wire or three-wire fan, wherein the fan control circuit and the fan control system are capable of controlling a rotational speed of the fan by detecting the rotational speed and integrating an external control signal using a signal integrating device.

2. Description of the Prior Art

Fans are widely applied in computer systems for heat dissipation. A fan can be divided into three types: four-wire, three-wire and two-wire. The four-wire fan has four pins: power, ground, tachometer and pulse width modulation (PWM) input, where the power pin is for providing power for the fan to operate, the tachometer pin is for detecting rotational speed information of the fan, and the PWM input pin controls rotational speed of the fan via a PWM signal. The three-wire fan has three pins: power, ground and tachometer, where the power pin replaces the function of PWM input, and controls rotational speed of the fan by varying the DC voltage, and the function of the tachometer pin is similar to that of the tachometer pin in the four-wire fan. The two-wire fan has two pins: power and ground, where the DC voltage of the power pin can also be varied to control the rotational speed of the fan. Since there is no tachometer pin in the two-wire fan, the rotational speed information of the fan cannot be measured and obtained by connecting a tachometer.

In a four-wire fan (which has both the PWM signal and the tachometer signal), the system may obtain the rotational speed information via the tachometer and control the rotational speed of the four-wire fan via the PWM signal. A feedback system is generated, where the PWM input pin and the power pin are utilized for controlling the rotational speed of the fan according to the rotational speed information obtained by the tachometer and an ambient temperature information, respectively. Please refer to FIG. 1, which is a schematic diagram of a conventional fan control system 10. As shown in FIG. 1, the fan control system 10 includes a four-wire fan 102, a DC voltage source 104, a temperature sensor 106, an external control module 108, a modulation converting module 110, a selection device 112 and a clock generator 114. The power pin of the four-wire fan 102 is coupled to the DC voltage source 104, and an input voltage V_(in) is supplied by the DC voltage source 104. The tachometer of the four-wire fan 102 is coupled to the external control module 108, in order to provide a rotational speed information FG. The PWM input pin is coupled to the clock generator 114, and a pulse width modulation signal PWM_(ctrl) generated by the clock generator 114 controls the rotational speed of the four-wire fan 102.

The fan control system 10 is formed by two loops. One of the loops is a temperature loop, where the temperature sensor 106 detects the temperature of the system to obtain an ambient temperature T_(a), and then outputs a temperature parametric signal V_(Temp) to the selection device 112. The selection device 112 then outputs a control signal V_(ctrl) to the clock generator 114. The clock generator 114 converts the control signal V_(ctrl) to the pulse width modulation signal PWM_(ctrl), in order to control the rotational speed of the four-wire fan 102. The other loop is an external control loop, where the external control module 108 obtains the rotational speed information FG, and then generates an external control signal PWM_(in). The modulation converting module 110 then converts the external control signal PWM_(in) in PWM form to an external control voltage signal V_(PWM) in voltage form to output to the selection device 112. Similarly, the selection device 112 outputs the control signal V_(ctrl) to the clock generator 114. The clock generator 114 converts the control signal V_(ctrl) to the pulse width modulation signal PWM_(ctrl), in order to control the rotational speed of the four-wire fan 102. As a result, the selection device 112 generates the control signal V_(ctrl) according to the temperature parametric signal V_(Temp) and the external control voltage signal V_(PWM) simultaneously.

Although the four-wire fan has more pins and more complete functions, the cost of the four-wire fan is higher than that of the two-wire or three-wire fan. If the two-wire or three-wire fan is required, the fan rotational speed cannot be controlled by the PWM input pin, and therefore the fan rotational speed cannot be controlled accurately by the system. Furthermore, in the four-wire fan control system 10, the DC voltage source 104 outputs the input voltage V_(in) with a fixed value to the four-wire fan 102. When other four-wire fans with different voltage specifications are utilized, the power supply has to be adjusted accordingly, which reduces the flexibility of utilization. Thus, there is a need for improvement over the prior art.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide a fan control circuit for controlling a two-wire or three-wire fan, which is capable of controlling the rotational speed of the two-wire or three-wire fan by detecting the rotational speed and integrating an external control signal via a signal integrating device.

The present invention discloses a fan control circuit for controlling a two-wire or three wire fan in a fan control system. The fan control circuit comprises: a rotational speed detecting module, for detecting a rotational speed of the fan, in order to generate a rotational speed signal; a rotational speed converting module, coupled to the rotational speed detecting module, for converting the rotational speed signal into a first voltage signal; and a feedback control module, coupled to the rotational speed converting module, for generating a fan control signal to control the fan according to the first voltage signal.

The present invention further discloses a fan control system, which comprises a two-wire or three-wire fan; and a fan control circuit for controlling the fan. The fan control circuit comprises: a rotational speed detecting module, for detecting a rotational speed of the fan, in order to generate a rotational speed signal; a rotational speed converting module, coupled to the rotational speed detecting module, for converting the rotational speed signal into a first voltage signal; and a feedback control module, coupled to the rotational speed converting module, for generating a fan control signal to control the fan according to the first voltage signal.

These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional fan control system.

FIG. 2 is a schematic diagram of a fan control system according to an embodiment of the present invention.

FIG. 3 is a schematic diagram of current variation caused by fan rotation.

FIG. 4 is a schematic diagram of an embodiment of the rotational speed signal and the external control signal generating the integrated voltage signal via the signal integrating device shown in FIG. 2.

FIG. 5A and FIG. 5B are schematic diagrams of correspondence between the duty cycle of the external control signal and the rotational speed of the two-wire fan according to different embodiments of the present invention.

FIG. 6 is a schematic diagram of an embodiment of the selection device and the DC-DC converter shown in FIG. 2.

DETAILED DESCRIPTION

Please refer to FIG. 2, which is a schematic diagram of a fan control system 20 according to an embodiment of the present invention. As shown in FIG. 2, the fan control system 20 includes a two-wire fan 202, a DC voltage source 204, a temperature sensor 206, an external control module 208 and a fan control circuit 250. The fan control circuit 250 includes a modulation converting module 210, a selection device 212, a DC-DC converter 214, a rotational speed detecting module 216, a rotational speed converting module 218 and a signal integrating device 220. The power pin of the two-wire fan 202 is coupled to the DC-DC converter 214, and the DC-DC converter 214 supplies an input voltage V_(in)′, where magnitude of the input voltage V_(in)′ determines rotational speed of the two-wire fan 202. Since the DC-DC converter 214 provides flexibility for voltage conversion between the input and output voltages, a voltage V_(CC) provided by the DC voltage source 204 is only required to be within a specific range (e.g. 12V-24V) for the fan control system 20 to operate normally. As a result, flexibility of fan selection and power supply can be significantly enhanced.

Functions of the temperature sensor 206 are similar to those of the temperature sensor 106. The temperature sensor 206 is utilized for obtaining an ambient temperature T_(a)′ and outputting a temperature parametric signal V_(Temp′). Functions of the external control module 208 are also similar to those of the external control module 108, where the main difference is that the external control module 208 cannot obtain the rotational speed information via the tachometer pin directly. Functions of the modulation converting module 210 are also similar to those of the modulation converting module 110. The modulation converting module 210 converts an external control signal PWM_(in)′ in pulse width modulation (PWM) form outputted by the external control module 208 into an external control voltage signal V_(PWM)′ in voltage form. The rotational speed detecting module 216 may replace the function of tachometer in the four-wire and three-wire fan, and can be utilized for detecting the rotational speed of the two-wire fan 202, in order to output a rotational speed signal S_(RS). The rotational speed detecting module 216 may detect the rotational speed of the two-wire fan 202 through various methods. In some embodiments, the rotational speed detecting module 216 may detect the rotational speed of the two-wire fan 202 via the Hall Effect. According to the principle of the Hall Effect, a rotating fan may drive the magnetic field to vary. If a voltage or current is exerted in the magnetic field, the variation of the magnetic field may cause the voltage or current to vary accordingly, and the variation may be detected to obtain the rotational speed signal S_(RS). For an illustration of this, please refer to FIG. 3, which is a schematic diagram of current variation caused by fan rotation. As shown in FIG. 3, when the fan rotates, periodic fluctuation may be generated in the current signal, and the cycle of this fluctuation will correspond to the cycle of the fan rotation. For example, a threshold value I_(th) may be set up. When the current passes through the threshold value I_(th), a signal will be inverted, and the rotational speed signal S_(RS) in clock form is thereby obtained. The method of detecting periodic fluctuation in the voltage signal to generate the rotational speed signal S_(RS) is similar to the method for the current signal, and will not be narrated herein.

The rotational speed converting module 218 converts the rotational speed signal S_(RS) into a rotational speed voltage signal V_(RS). The signal integrating device 220 receives both the rotational speed voltage signal V_(RS) and the external control voltage signal V_(PWM)′, and generates an integrated voltage signal V_(PWM) _(—) _(RS). The selection device 212 outputs a control signal V_(ctrl)′ to the DC-DC converter 214 according to the temperature parametric signal V_(Temp)′ and the integrated voltage signal V_(PWM) _(—) _(RS). The DC-DC converter 214 then generates a corresponding input voltage V_(in)′ allowing the two-wire fan 202 to control the rotational speed of the two-wire fan 202.

Please note that the external control module 208 may not obtain the rotational speed information via the tachometer directly, as mentioned above. The external control module 208, however, still has to perform accurate control through other methods according to the rotational speed information of the two-wire fan 202. Please refer to FIG. 4, which is a schematic diagram of an embodiment of the rotational speed signal S_(RS) and the external control signal PWM_(in)′ generating the integrated voltage signal V_(PWM) _(—) _(RS) via the signal integrating device 220 shown in FIG. 2. As shown in FIG. 4, the signal integrating device 220 can be realized by an operational amplifier. According to characteristics of the operational amplifier, the rotational speed voltage signal V_(RS) received by the positive input terminal of the operational amplifier and the external control voltage signal V_(PWM)′ received by the negative input terminal of the operational amplifier are equal, such that the modulation converting module 210 can adjust a relationship between the external control voltage signal V_(PWM)′ and the external control signal PWM_(in)′, and thereby control the two-wire fan 202 via the external control signal PWM_(in)′ according to the rotational speed signal S_(RS).

In detail, the rotational speed converting module 218 converts the rotational speed signal S_(RS) into the rotational speed voltage signal V_(RS) via a conversion table 418. The conversion table 418 may set the rotational speed signal S_(RS) and the rotational speed voltage signal V_(RS) to be in a direct ratio. The relationship between these two signals may also be set up in other manners according to system requirements, which is not limited herein. In some embodiments, a transistor 402 maybe coupled to an output terminal of the rotational speed detecting module 216, in order to output the rotational speed signal S_(RS). The two-wire fan 202 is monitored externally, in order to detect whether the two-wire fan 202 operates normally. When an abnormal condition occurs in the rotational speed of the two-wire fan 202, the system may control the two-wire fan 202 to stop operating, or turn the fan off. The modulation converting module 210 includes a comparator 404, a resistor R1 and a capacitor C1. The comparator 404 includes an input terminal coupled to the external control module 208, another input terminal which receives a reference voltage V_(ref), and an output terminal coupled to the signal integrating device 220. The reference voltage V_(ref) may be set to a middle voltage between a higher voltage level and a lower voltage level of the external control signal PWM_(in)′, such that the comparator 404 may perform charging or discharging on the external control voltage signal V_(PWM)′ according to whether the external control signal PWM_(in)′ is located in the higher voltage level or the lower voltage level. The resistor R1 and the capacitor C1 may be utilized for setting a correspondence between the duty cycle and the external control voltage signal V_(PWM)′, in order to set a maximum value of the external control voltage signal V_(PWM)′ outputted to the signal integrating device 220 (i.e. to control the maximum value of the external control voltage signal V_(PWM)′ to correspond to a 100% duty cycle). The external control signal PWM_(in)′ is converted into a current output via the comparator 404 and then converted into the external control voltage signal V_(PWM)′ via the resistor R1; parameters of the comparator 404 or the resistor R1 can thereby be adjusted, in order to adjust a correspondence between the duty cycle and voltage in a conversion table 410. In an embodiment, the resistor R1 may be placed off-chip and designed as a variable resistor, so that the resistor R1 can be adjusted easily. Most commercially available fans have a minimum rotational speed; hence in some embodiments, a minimum rotational speed setting module 406 can be coupled to the signal integrating device 220, in order to clamp a lower limit of the external control voltage signal V_(PWM)′, which further limits the minimum rotational speed of the two-wire fan 202.

According to the correspondence between the duty cycle of the external control signal PWM_(in)′ and the external control voltage signal V_(PWM)′ in the conversion table 410, and the correspondence between the rotational speed signal S_(RS) and the rotational speed voltage signal V_(RS) in the conversion table 418, a correspondence between the duty cycle of the external control signal PWM_(in)′ and the rotational speed of the two-wire fan 202 may be obtained. Please refer to FIG. 5A and FIG. 5B, which are schematic diagrams of correspondence between the duty cycle of the external control signal PWM_(in)′ and the rotational speed of the two-wire fan 202 according to different embodiments of the present invention. As shown in FIG. 5A and FIG. 5B, a larger duty cycle corresponds to a higher rotational speed, and 100% duty cycle corresponds to the highest rotational speed. The difference between FIG. 5A and FIG. 5B is that, in FIG. 5A, when the duty cycle is reduced to smaller than P%, the minimum rotational speed setting module 406 further controls the two-wire fan 202 to remain at the minimum rotational speed. In comparison, in FIG. 5B, when the duty cycle is reduced to 0%, the rotational speed of the two-wire fan 202 is reduced to the minimum.

The selection device 212 outputs the control signal V_(ctrl)′ according to the temperature parametric signal V_(Temp)′ and the integrated voltage signal V_(PWM) _(—) _(RS). The DC-DC converter 214 then generates the input voltage V_(in)′ to provide for the two-wire fan 202 according to the control signal V_(ctrl)′, in order to control the rotational speed of the two-wire fan 202. Please refer to FIG. 6, which is a schematic diagram of an embodiment of the selection device 212 and the DC-DC converter 214 shown in FIG. 2. The selection device 212 includes diodes D1 and D2 for receiving the integrated voltage signal V_(PWM) _(—) _(RS) and the temperature parametric signal V_(Temp)′, respectively, in order to select one of the integrated voltage signal V_(PWM) _(—) _(RS) and the temperature parametric signal V_(Temp)′ according to which has a larger voltage as the control signal V_(ctrl)′. In a general condition, the rotational speed of the two-wire fan 202 is controlled by the external control module 208 according to system requirements. The integrated voltage signal V_(PWM) _(—) _(RS) is mainly the control signal V_(ctrl)′. When system temperature rises rapidly or rises to an over-value, however, the temperature sensor 206 may react rapidly and output a higher temperature parametric signal V_(Temp)′. When the temperature parametric signal V_(Temp)′ is greater than the integrated voltage signal V_(PWM) _(—) _(RS) the temperature parametric signal V_(Temp)′ is outputted as the control signal V_(ctrl′) instead. This controls the rotational speed of the two-wire fan 202 to rise, for reducing the temperature rapidly. The DC-DC converter 214 includes an operational amplifier 602 and a control circuit 604. The control circuit 604 may operate in a PWM mode or pulse frequency modulation (PFM) mode, and can be utilized for controlling the DC-DC converter 214 to generate the input voltage V_(in)′ to provide for the two-wire fan 202 according to the control signal V_(ctrl)′, in order to control the rotational speed of the two-wire fan 202. The method of the DC-DC converter 214 generating the input voltage V_(in)′ according to the control signal V_(ctrl)′ should be well-known by those skilled in the art, and will not be narrated herein.

The spirit of the present invention is to control the rotational speed of the two-wire or three-wire fan by detecting the rotational speed of the fan and by integrating an external control signal via a signal integrating device. Those skilled in the art can make modifications and alterations accordingly. For example, in the conversion table shown in FIG. 5A and FIG. 5B, the larger duty cycle corresponds to the higher rotational speed. In other embodiments, however, the larger duty cycle may correspond to the lower rotational speed, and its related implementation may also correspond to the embodiment shown in FIG. 4, where two input terminals of the comparator 404 are interchanged. In some embodiments, multiple external control modules may also be utilized together with the modulation converting module 210, or multiple rotational speed detecting modules may be utilized together with the rotational speed converting module 218, in order to realize the application of multiple fans. In addition, the two-wire fan 202 is utilized in the fan control system 20 according to the above embodiment, but in other embodiments, three-wire fans may also be utilized in the fan control system 20, which is not limited herein. For the three-wire fans, however, the rotational speed should be detected by using tachometers, which is different from the abovementioned detecting method realized by the rotational speed detecting module 216 utilizing the magnetic field to drive the varied voltage or current. Other devices in the above embodiment may also be utilized for the three-wire fans to achieve fine control.

In the prior art, the cost of the four-wire fan is higher than that of the two-wire or three-wire fan. If the two-wire or three-wire fan with lower cost is required, the fan rotational speed cannot be controlled by the PWM input pin, such that the fan rotational speed cannot be controlled accurately by the system. In the conventional four-wire fan control system, the DC voltage source outputs a fixed input voltage. When other four-wire fans with different voltage specification are utilized, the power supply has to be adjusted accordingly, which reduces the flexibility of utilization. In comparison, the fan control systems according to the present invention are capable of controlling a rotational speed of the two-wire or three-wire fan by detecting the rotational speed and integrating an external control signal via a signal integrating device. In addition, the DC-DC converter 214 provides flexibility for voltage conversion between the input and output voltages, such that flexibility of fan selection and power supply can be significantly enhanced. In comparison with the conventional fan control system, the present invention has significant improvements in terms of cost and utilization flexibility.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims. 

What is claimed is:
 1. A fan control circuit for controlling a two-wire or three wire fan in a fan control system, the fan control circuit comprising: a rotational speed detecting module, for detecting a rotational speed of the fan, in order to generate a rotational speed signal; a rotational speed converting module, coupled to the rotational speed detecting module, for converting the rotational speed signal into a first voltage signal; and a feedback control module, coupled to the rotational speed converting module, for generating a fan control signal to control the fan according to the first voltage signal.
 2. The fan control circuit of claim 1, further comprising a DC-DC converter, for converting the fan control signal to output a signal corresponding to a voltage specification of the fan.
 3. The fan control circuit of claim 1, wherein the rotational speed signal is in a voltage form or a current form.
 4. The fan control circuit of claim 1, wherein the fan control system further comprises a temperature sensor, for detecting an ambient temperature of the fan, in order to generate a temperature parametric signal.
 5. The fan control circuit of claim 4, wherein the feedback control module is further coupled to the temperature sensor, in order to generate the fan control signal according to the temperature parametric signal.
 6. The fan control circuit of claim 1, wherein the fan control system further comprises an external control module, for generating an external control signal to control the fan control circuit.
 7. The fan control circuit of claim 6, wherein the external control signal is a pulse width modulation signal, and the fan control circuit further comprises a modulation converting module for converting the pulse width modulation signal into a second voltage signal.
 8. The fan control circuit of claim 7, wherein the feedback control module is further coupled to the modulation converting module, in order to generate the fan control signal according to the second voltage signal.
 9. The fan control circuit of claim. 8, wherein the fan control system further comprises a temperature sensor, for detecting an ambient temperature of the fan, in order to generate a temperature parametric signal.
 10. The fan control circuit of claim 9, wherein the feedback control module is further coupled to the temperature sensor, in order to generate the fan control signal according to the temperature parametric signal.
 11. The fan control circuit of claim 10, wherein the feedback control module comprises: a signal integrating device, for integrating the first voltage signal and the second voltage signal to generate a third voltage signal; and a selection device, for selecting one of the temperature parametric signal and the third voltage signal according to which has a greater voltage as a fourth voltage signal, and generating the fan control signal via the fourth voltage signal.
 12. The fan control circuit of claim 11, wherein the signal integrating device comprises an operational amplifier, which comprises: a first input terminal, coupled to the rotational speed converting module, for receiving the first voltage signal; a second input terminal, coupled to the modulation converting module, for receiving the second voltage signal; and an output terminal, coupled to the selection device, for outputting the third voltage signal.
 13. The fan control circuit of claim 7, wherein the modulation converting module adjusts a correspondence between the pulse width modulation signal and the second voltage signal according to a correspondence between the first voltage signal and the second voltage signal.
 14. The fan control circuit of claim 1, wherein the fan control circuit further comprises a minimum rotational speed setting module, for limiting a minimum rotational speed of the fan.
 15. A fan control system, comprising: a two-wire or three-wire fan; and a fan control circuit, for controlling the fan, the fan control circuit comprising: a rotational speed detecting module, for detecting a rotational speed of the fan, in order to generate a rotational speed signal; a rotational speed converting module, coupled to the rotational speed detecting module, for converting the rotational speed signal into a first voltage signal; and a feedback control module, coupled to the rotational speed converting module, for generating a fan control signal to control the fan according to the first voltage signal.
 16. The fan control system of claim 15, further comprising a DC-DC converter, for converting the fan control signal to output a signal corresponding to a voltage specification of the fan.
 17. The fan control system of claim 16, wherein the rotational speed signal is in a voltage form or a current form.
 18. The fan control system of claim 15, further comprising a temperature sensor, for detecting an ambient temperature of the fan, in order to generate a temperature parametric signal.
 19. The fan control system of claim 18, wherein the feedback control module is further coupled to the temperature sensor, in order to generate the fan control signal according to the temperature parametric signal.
 20. The fan control system of claim 15, further comprising an external control module, for generating an external control signal to control the fan control circuit.
 21. The fan control system of claim 20, wherein the external control signal is a pulse width modulation signal, and the fan control circuit further comprises a modulation converting module for converting the pulse width modulation signal into a second voltage signal.
 22. The fan control system of claim 21, wherein the feedback control module is further coupled to the modulation converting module, in order to generate the fan control signal according to the second voltage signal.
 23. The fan control system of claim 22, further comprising a temperature sensor, for detecting an ambient temperature of the fan, in order to generate a temperature parametric signal.
 24. The fan control system of claim 23, wherein the feedback control module is further coupled to the temperature sensor, in order to generate the fan control signal according to the temperature parametric signal.
 25. The fan control system of claim 24, wherein the feedback control module comprises: a signal integrating device, for integrating the first voltage signal and the second voltage signal to generate a third voltage signal; and a selection device, for selecting one of the temperature parametric signal and the third voltage signal according to which has a greater voltage as a fourth voltage signal, and generating the fan control signal via the fourth voltage signal.
 26. The fan control system of claim 25, wherein the signal integrating device comprises an operational amplifier, which comprises: a first input terminal, coupled to the rotational speed converting module, for receiving the first voltage signal; a second input terminal, coupled to the modulation converting module, for receiving the second voltage signal; and an output terminal, coupled to the selection device, for outputting the third voltage signal.
 27. The fan control system of claim 21, wherein the modulation converting module adjusts a correspondence between the pulse width modulation signal and the second voltage signal according to a correspondence between the first voltage signal and the second voltage signal.
 28. The fan control system of claim 15, wherein the fan control circuit further comprises a minimum rotational speed setting module, for limiting a minimum rotational speed of the fan. 